Off-axis re-entrant cavity ring-down spectroscopy with a mid-infrared continuous-wave optical parametric oscillator

We demonstrate an off-axis cavity ring-down spectroscopy system, which uses a mid-infrared continuous- wave optical parametric oscillator as a light source. Off-axis injectionwith re-entrant configuration of the ring-down cav- ity is used to achieve high spectral resolution while main- taining high measurement speed. This makes the setup suit- able for sensitive molecular spectroscopy in the mid-infrared region, particularly for studies that require high temporal resolution. Formaldehyde (H2CO) absorption spectrum at 3.4 µmismeasured using the off-axis re-entrant cavity ring- down spectrometer.